DUBAI (Reuters) - Islamic State militants fighting in Iraq and Syria released a video on Tuesday that appears to show British journalist John Cantlie criticizing preparations for U.S-led attacks on the militant group, the SITE monitoring service reported.

Islamic State, which controls territory in Syria and Iraq, has already executed two U.S. journalists and one British aid worker in recent weeks in what it said was reprisal for U.S. air strikes against it in Iraq.

In the five-minute video, the man identified as Cantlie suggests President Barack Obama, long careful to avoid the sort of conflicts his predecessor George Bush pursued, was being sucked into a war he could not win, SITE reported.

"The president once called George Bush's Iraq conflict a "dumb war," and couldn't wait to distance America from it when he came into power. Now he's being inextricably drawn back in," the man identified as Cantlie says.

The man, wearing an orange shirt and his hair closely cropped, describes Islamic State as the "most powerful jihadist movement seen in recent history", adding it could not be greatly harmed by U.S. politicians calling it "awful" or "vile".

The video appeared to have been recorded before strikes overnight launched by U.S. warplanes and partners on Islamic State targets in Syria.

The United States has been building a coalition to combat Islamic State, a hardline Sunni Muslim force that has seized large expanses of territory in Iraq and Syria and proclaimed a caliphate erasing borders in the heart of the Middle East.

The United States resumed air strikes in neighboring Iraq in August for the first time since the pullout of U.S. troops in 2011.

Using a term for holy warriors, the man identified as Cantlie said: "Not since Vietnam have we witnessed such a potential mess in the making. Current estimates of 15,000 troops needed to fight the Islamic State are laughably low. The State has more mujahideen than this.

"This is not some undisciplined outfit with a few Kalashnikovs."

The man said the new Iraqi government, an ally of Shi'ite Muslim power Iran, was waiting eagerly for U.S. intervention to strengthen Iranian influence in the Middle East.

While a strong opponent of Islamic State, which sees Shi'ites as infidels, Iran has sent mixed signals about its willingness to cooperate with the United States on defeating the militants.

In public, both Washington and Tehran have ruled out cooperating militarily on Islamic State. But in private, Iranian officials have voiced a willingness to work with Washington on IS, though not necessarily on the battlefield.